Process for producing mixed esters of fatty acids as biofuels

ABSTRACT

A process for producing mixed esters of fatty acids as biofuel or additive to a petroleum fuel for use in a compression ignition (CI) engine. The process preferably provides a partial transesterification of a mixture of fatty acid methyl esters with at least one alkyl alcohol containing 2 to 8 carbon atoms in the presence of a heterogeneous solid acid catalyst to produce a mixture of the fatty acid methyl esters and alkyl alcohol esters of the fatty acids.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed to Provisional Application No. 61/003,790, filedNov. 20, 2007, the entire disclosure of which is herein incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to a process producing mixedesters of fatty acids as biofuels particularly for use in a compressionignition (CI) (particularly diesel) engine.

(2) Description of Related Art

Biodiesel has been considered as an alternative to petroleum baseddiesel for many years. Biodiesel is a general term referring to a fuelcomprising methyl-esters of long chain fatty acids derived typicallyfrom vegetable oils or animal fats. It can be used per se as fuel, or asan additive in a blend with petroleum-based diesel fuel.

The biodiesel industry in the U.S. currently suffers from a lack ofconsumer confidence in the quality of the biofuel. The uncertainty infuel quality stems from lack of experience on the part of producers inbeing consistent in their production methods, and the all-too-oftenabsence of careful fuel analysis to ensure that quality standards suchas ASTM specifications are achieved. Although several measures ofbiodiesel quality can be used, the limiting factor is often thecold-weather performance properties of biodiesel, manifested as a cloudpoint and pour point temperature that is too high for the climate. Ahigh cloud point temperature in biodiesel is typically observed becauseof (1) glyceride impurities present and (2) the presence of only methylesters of unsaturated and saturated fatty acids. If residual glycerideimpurities are not removed from the finished product, the biodieselforms crystals at low temperature and those crystals plug fuel filtersand injectors. Even if the impurities are removed, the saturated fattyacid methyl esters crystallize at some point as temperature is reduced,thus leading to solids formation. Thus, auto and diesel enginemanufacturers at present will only warrant biofuel compositions up to abiofuel content of B5 (5% biodiesel/95% petroleum). Ideally, a highquality biofuel for the North American climate would contain at leastB20 (20% biodiesel, 80% petroleum).

Biodiesel offers several advantages. In particular, when compared topetroleum diesel, biodiesel provides similar fuel economy, horsepowerand torque while providing superior lubricity. Moreover, biodieselprovides a substantial reduction of emission of unburned hydrocarbons,carbon monoxide, and particulate matter. Typically, it is free of sulfurand aromatics which are major pollutants. Accordingly, biodiesel isconsidered a renewable, non-toxic and biodegradable fuel alternative oradditive.

Previous methods associated with producing mixed esters of fatty acidsinclude reaction of (1) a mixture of different triglycerides withmethanol; or (2) an oil (triglycerides) with a mixture of differentalcohols, namely ethanol, methanol, n-butanol and n-propanol, in thepresence of a base catalyst chosen from sodium hydroxide, potassiumhydroxide and sodium methylate. In presence of a base catalyst (sodiumor potassium hydroxides), the rate and extent of ester formation aredirectly proportional to the formation of sodium or potassium alkoxidefrom the alcohol in situ in the reaction mixture.

Upon completion of the reaction, ideally two distinct phases, 1)glycerol (a trihydroxy alcohol) and 2) esters of fatty acids, areobserved. The following limitations are typically associated with thisprocess: 1) the transesterification reaction proceeds smoothly only whenmethanol is used as an alcohol and fatty acid methyl ester (FAME) issynthesized, and the reaction is adversely affected when higher alcoholssuch as ethanol, n-propanol and n-butanol are used; 2) use of higheralcohols also creates a problem of readily separating glycerol from thealkyl esters of fatty acids, which requires additional processing stepsincluding alcohol separation from the reaction mixture and dilute acidwash to facilitate glycerol phase separation from fatty acid ester; and3) use of base catalyst increases the prospect of soap formation(saponification of the fatty acids) which is quite detrimental tooverall process and its economy. It is a further disadvantage ofexisting processes that the base catalysts used in the reaction systemare not re-usable, thereby generating a considerably significantquantity of salt waste.

U.S. Patent Application No. 2007/0056213 to French et al. describes amethod which includes operating a two-stroke engine with a lubricatingfuel. A fuel/lubricant formulation is disclosed for the operation of thetwo-stroke engines with improved emissions and performance. Thelubricating fuel includes at least one fuel selected from the groupincluding C1-6 alcohol, gasoline, ether, ketone, nitromethane, and amixture thereof, and at least one lubricant selected from the groupincluding biodiesel, lipid fatty acid alkyl ester, fatty acid and amixture thereof. Diesel fuels are not described.

Patent application WO 2006/107407 to Clements describes processes andsystems for producing biodiesel or fatty acid esters from multipletriglyceride feedstocks using a two step reaction with an alcohol andacid catalyst and then an alkaline catalyst. The first step forms anacid alcohol layer and an ester-triglyceride layer. The second stepreacts the ester triglyceride layer with a base to form the fatty acidesters.

Patent application WO 2006/128881 to Despeghel describes alkyl-estercompositions derived from rapeseed and sunflower, in particular fromBrassica napus and Helianthus annuus using an acid catalyst in a batchprocess. Despeghel further discloses a process for preparation of thealkyl-esters. The alkyl-ester compositions can be used in diesel enginesin its pure form or blended with another composition such as fuel.

U.S. Pat. No. 6,468,319 to Yeh et al. describes various esters used indiesel fuel to reduce emissions. U.S. Pat. No. 5,268,008 to Kannedescribes esters used in diesel fuels to reduce emissions.

While the related art teach processes for generating biodiesel, therestill exists a need for improved processes and compositions forgenerating biodiesel.

OBJECTS

Therefore, it is an object of the present invention to provide abiodiesel composition and an improved process for generating thecomposition. The process produces an improved biofuel which comprises inpart or in whole mixed alkyl esters of fatty acids. The inventionprovides a process that can be added to current biodiesel productionfacilities, with the goal of removing impurities from the biodieselstream and at the same time producing a mixed alkyl ester fuel withsuperior properties.

These and other objects of the present invention will becomeincreasingly apparent with reference to the following drawing andpreferred embodiments.

SUMMARY OF THE INVENTION

The present invention provides a continuous process for producing afatty acid alkyl ester mixture which comprises: (a) countercurrentlyreacting a fatty acid methyl ester at a temperature between about 50 and200° C. and a pressure between about 0.5 and 20 atmospheres with atleast one alkyl alcohol having 2 to 8 carbon atoms in the presence of asolid acid catalyst in a reactive zone in a distillation column toproduce a fatty acid alkyl ester mixture; and (b) recovering theproduced fatty acid alkyl ester mixture at the bottom of the column andthe alcohol from the top of the column. In further embodiments, thesolid acid catalyst is a heterogeneous acid catalyst. In further stillembodiments, the reactive distillation column comprises: (a) thereactive zone containing the solid acid catalyst; (b) a fatty acid esterinlet above the catalyst; (c) an alcohol inlet below the catalyst; (d)product outlet at the bottom of the column for separating the producedfatty acid alkyl ester; and (e) an outlet at the top of the column formethanol formed in the reaction and at least one alcohol which isunreacted. In still further embodiments, the solid catalyst is aheterogeneous solid acid catalyst. In further still embodiments, thefatty acid methyl ester comprises a mixture of fatty acid methyl estersand methanol, monoglyceride, diglyceride, triglyceride, and glycerol asimpurities. In further still embodiments, the present disclosureprovides for a process wherein at least the monoglyceride, diglycerideand triglyceride of the impurities are transesterified in the reactionwith the alkyl alcohol having 2 to 8 carbon atoms producing mixed fattyacid alkyl esters to be separated at the bottom of the column, alongwith glycerol. The glycerol leaving the column at the bottom can be atleast partially removed by a process selected from the group consistingof phase separation, water washing, and application of an adsorbent.

In an exemplary embodiment, the fatty acid methyl ester is generatedfrom a preliminary transesterification reaction of a vegetable oil as atriglyceride with methanol. In further still embodiments, thepreliminary transesterification reaction of a vegetable oil as atriglyceride with methanol takes place in a biodiesel production plantand the reactive distillation column is added onto the biodieselproduction plant to receive the fatty acid methyl ester product stream.In further still embodiments, the fatty acid alkyl ester produced is inadmixture with the fatty acid methyl ester. In still furtherembodiments, the alkyl alcohol is ethanol and the fatty acid alkyl esteris a fatty acid ethyl ester. In further still embodiments, the alkylalcohol is a mixture of alcohols having 2 to 8 carbon atoms. In stillfurther embodiments, the alkyl alcohol mixture contains ethanol andbutanol and the fatty acid alkyl ester produced comprises a mixture of afatty acid ethyl ester and a fatty acid butyl ester. In further stillembodiments, the alkyl alcohol is selected from the group consisting ofethanol, propanol, isopropanol, butanol, isobutanol, amyl and iso-amylalcohol and mixtures thereof. In still further embodiments, the producedfatty acid alkyl ester has a cloud point lower in temperature than themethyl fatty acid ester. In further still embodiments, the producedfatty acid alkyl ester is in addition blended with a petroleum dieselfuel. In still further embodiments, the petroleum diesel fuel is 50 to95% of the blend. In still further embodiments, the fatty acid methylester is selected from the group consisting of methyl palmitate, methylstearate, methyl oleate, methyl linoleate, methyl linolenate, andmixtures thereof. In further still embodiments, the produced fatty acidalkyl ester is derived from the group of acids selected from the groupconsisting of stearic acid, oleic acid, linoleic acid, linolenic acid,and mixtures thereof.

The present invention further provides a process for preparing a fattyacid ester mixture, useful alone or in combination with petroleum fuel,in a compression ignition engine, which comprises: (a) reacting animpure fatty acid methyl ester mixture with at least one alkyl alcoholcontaining 2 to 8 carbon atoms in the presence of a base or acidwherein, the impure fatty acid methyl ester mixture comprises fatty acidmethyl ester and methanol, one or more of a monoglyceride, adiglyceride, and a triglyceride as impurities; wherein the reactionpartially transesterifies the impure methyl ester mixture to produce amixture of fatty acid alkyl esters containing 1 to 8 carbon atoms; and(b) removing methanol formed in the reaction and unreacted alkyl alcoholto produce the fatty acid ester mixture. In an exemplary embodiment, thealcohol is ethanol. In further still embodiments, at least one alcoholis ethanol and at least one of the produced alkyl fatty acid esters is afatty acid ethyl ester. In still further embodiments, the impure mixtureis reacted with a mixture of two or more alcohols at least one of whichis ethanol. In further still embodiments, the mixture of alcoholscomprises ethanol and butanol and the reaction with the impure fattyacid methyl ester mixture further produces a fatty acid ethyl ester anda fatty acid butyl ester.

The present invention provides for an exemplary process for producingmixed esters of fatty acids, which comprises: (a) partiallytransesterifying at a temperature between about 50 and 200° C. and apressure between about 0.5 and 20 atmospheres, a mixture of fatty acidmethyl esters with at least one alkyl alcohol containing 2 to 8 carbonatoms in the presence of a heterogeneous solid acid catalyst to producea mixture of the fatty acid methyl esters and alkyl alcohol derivedfatty acid esters; and (b) separating the esters from the catalyst. Infurther embodiments, the mixture of fatty acid methyl esters comprisesof methyl palmitate, methyl stearate, methyl oleate, methyl linoleateand methyl linolenate. In still further embodiments, the alkyl alcoholis selected from the group consisting of ethanol, n-propanol, n-butanol,iso-amyl alcohol and mixtures thereof.

The present invention provides for a composition useful in a compressionignition (CI) engine which comprises a mixture of fatty acid methylesters and of fatty acid alkyl esters of at least one alkyl alcoholcontaining 2 to 8 carbon atoms, wherein the cloud point of the mixtureis lower in temperature than that of the mixture of the fatty acidmethyl esters alone. In further embodiments, the mixture of fatty acidmethyl esters comprises esters of palmitic acid, stearic acid, oleicacid, linoleic acid and linolenic acid from soybean.

The present invention provides for a composition adapted for use in acompression ignition (CI) engine, comprising a mixture of fatty acidmethyl esters, fatty acid alkyl esters of at least one alkyl alcoholcontaining 2 to 8 carbon atoms, esters of a fermentation ornon-fermentation derived organic acid with at least one alkyl alcoholcontaining 1 to 6 carbon atoms and optionally an ether containing atleast 6 carbon atoms as an oxygenate. In further embodiments, a cloudpoint of the mixture is at a lower in temperature than that of the fattyacid methyl esters alone. In further still embodiments, the mixture offatty acid methyl esters comprises methyl palmitate, methyl stearate,methyl oleate, methyl linoleate and methyl linolenate. In still furtherembodiments, the alkyl alcohol is selected from the group consisting ofethanol, propanol, butanol, iso-amyl alcohol and mixtures thereof. Infurther still embodiments, the mixture of fatty acid methyl and alkylesters comprises esters of palmitic acid, stearic acid, oleic acid,linoleic acid, dimethyl succinate and linolenic acid. In still furtherembodiments, the fermentation or non-fermentation derived organic acidester is derived from an acid selected from the group consisting oflactic acid, succinic acid and mixtures thereof. In still furtherembodiments the fermentation or non-fermentation derived organic acidester is derived from an acid selected from the group consisting ofpropionic acid, butyric acid, isobutyric acid, dicarboxylic andcarboxylic acids, and mixtures thereof. In still further embodiments,the ether is selected from the group consisting of dibutyl ether (DBE);methyl tert-butyl ether (MTBE); tertiary amyl methyl ether (TAME);tertiary hexyl methyl ether (THEME); ethyl tertiary butyl ether (ETBE);tertiary amyl ethyl ether (TAEE); propyl ether (DIPE); dipropyl ether;dihexyl ether; dioctyl ether, and iso-amyl-ether. In further stillembodiments, the oxygenate is an ester.

An exemplary embodiment associated with the present invention providesfor a biofuel, for use in compression ignition (CI) engines, comprisingmixtures of fatty acid methyl esters, fatty acid esters of at least onealcohol containing 2 to 8 carbon atoms, esters of a fermentation derivedorganic acid with at least one alcohol containing 1 to 6 carbon atomsand optionally an ether containing at least 6 carbon atoms as anoxygenate. In an exemplary embodiment, the fermentation derived organicacid can be derived from a carbohydrate (such as sugar) fermentationsource. The cloud point of the mixture can be lower in temperature thanthat of the fatty acid methyl esters alone (conventionally known asbiodiesel and currently used in CI engines). Typically, the mixture offatty acid methyl esters contains methyl palmitate, methyl stearate,methyl oleate, methyl linoleate and methyl linolenate. The alcohol canbe selected from the group consisting of ethanol, propanol, butanol,iso-amyl alcohol and mixtures thereof. The mixed fatty acid esters aremethyl and higher alcohol esters of palmitic acid, stearic acid, oleicacid, linoleic acid and linolenic acid. The fermentation derived organicacid can be selected from the group consisting of lactic acid, succinicacid and mixtures thereof. In a further exemplary embodiment, theorganic acid can be selected from the group consisting of propionicacid, butyric acid, isobutyric acid, mixed carboxylic acids and mixturesthereof. These can be used as additional fuel additives. In an exemplaryembodiment, the ether is dibutyl ether (DBE).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the continuous process of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

All patents, patent applications, government publications, governmentregulations, and literature references cited in this specification arehereby incorporated herein by reference in their entirety. In case ofconflict, the present description, including definitions, will control.

The present invention relates to (1) the preparation of mixed esters offatty acids and (2) mixtures of fatty acid methyl esters and diethylsuccinate/ ethyl lactate (a succinic acid ester/ lactic acid ester), tobe used as fuel in diesel engines. In an exemplary embodiment, mixedesters of fatty acids and blended FAME, a commercially available impurefatty acid methyl ester, have cloud points 5-10° C. lower than FAME(defined as Biodiesel by National Biodiesel Board). The lower cloudpoints of these mixed esters can make them more suitable than currentbiodiesel for use as a diesel fuel, particularly without the use ofchemical additives. It is a further aspect of the present invention thatthe exemplary mixed ester compositions can be blended in higherconcentrations into petroleum diesel than standard FAME.

It should be understood by those having skill in the art that a fattyacid is generally described as a carboxylic acid having an aliphatictail (chain), either saturated or unsaturated and typically unbranched.Carboxylic acids as short as butyric acid (4 carbon atoms) areconsidered to be fatty acids. Fatty acids derived from natural fats andoils usually have at least 8 carbon atoms, e.g. caprylic acid (octanoicacid).

Most natural fatty acids have an even number of carbon atoms, becausetheir biosynthesis involves acetyl-CoA, a coenzyme carrying atwo-carbon-atom group. Fatty acids are typically produced by thehydrolysis of the ester linkages in a fat or biological oil (both ofwhich are triglycerides), with the production of glycerol. FAME (FattyAcid Methyl Ester) is generally characterized by the chemical formula,ROOCH3, wherein R is a fatty acid carbon chain.

As used herein, the term “higher alkyl alcohols (ROH)” refers toalcohols having 2 to 8 carbon atoms (e.g., as compared to methanol). Asused herein, a lower alkyl alcohol is an alcohol with only one carbonsuch as methanol, but can be ethanol, propanol, or butanol if reactedwith a higher alkyl ester, wherein the alkyl has more carbons than thealcohol (i.e., carbons of alkyl are greater than carbons of alcohol).However, it is understood that the terms “lower” and “higher” arerelative terms based on the carbon atoms of the alcohol and the alkylgroup of the reacting ester. For example, ethanol is a higher alkylalcohol when reacted with a lower-methyl-ester. In another example,butanol is a higher alkyl alcohol when reacted with a lower-ethyl-ester.

Exemplary mixed esters according to the present invention aresynthesized using a transesterification process. The term“transesterification” refers to a process of exchanging the lower alkylgroup of an ester compound with a higher alkyl group or vice versa.These reactions are often catalyzed by the addition of an acid or base.

Example:

-   -   Higher alkyl Alcohol+lower alkyl ester→lower alcohol+higher        alkyl ester

Solid acid catalysts donate a proton to the carbonyl group, thus makingit more reactive.

The transesterification reactions associated in the Examples below werecarried out using several exemplary catalysts including but not limitedto: sodium ethylate (base), Amberlyst-15 (Bronsted acid), Amberlyst-70(acid), SBA-15/titanium diisoprpyl bis(acac)₂, SBA-15 (acid),SBA-15/dibutyltin bis(acac)₂ (acid),SBA-15/aminopropyltrimethoxysilnae-titanium tetrachloride (Lewis acid),SBA-15/ruthenium(acac)₂, and sulfated zirconia (acid) catalysts. Thesodium ethylate (NaOC2H5—also known as sodium ethoxide) is a base thatoperates as a catalyst. Generally, the base catalyst is not consumed inthe reaction unless free fatty acid is present. It is desirable to avoidthe presence of free fatty acid. Preferably the reaction mixture issubstantially free of free fatty acids. The presence of free fatty acidswith the sodium ethylate base catalyst forms a salt of the free acid.This process is saponification and results in the formation of soap. Inan exemplary embodiment, the reaction mixture contains less than 1.0%-wtof free fatty acid, more preferably less than 0.1%-wt of free fattyacid, and even more preferably less than 0.01%-wt of free fatty acid.The term “acac” associated with the catalysts in Experiment Nos. 8, 9,and 11 in Table 1 below refers to “acetyl acetate”, a common organicligand used to make homogeneous complexes of transition metals that aresoluble in water. SBA-15 is a mesoporous silica-alumina solid acid thatserves as a catalyst support for other oxide catalysts or as a catalystitself. It is related to zeolites in its composition and acidity. It isa common material known in the art. Amberlyst 15 and Amberlyst 70 arestrong cationic exchange resins. Typically, Amberlyst 70 can toleratehigher temperatures (150° C. vs. 120° C. for Amberlyst 15), but at acost of approximately ⅔ of the active (acid) site concentration per unitmass. Sulfated zirconia is ZrO₂ that has been treated with sulfuric acidto produce —SO₃ groups on the surface. These are highly acid groups thatgive sulfated zirconia its high catalyst activity. Typically, thecatalysts that are placed onto the SBA-15 support are organometalliccomplexes of varying composition that are known to be active acidcatalysts. In an exemplary embodiment, FAME was used as the startingsubstrate for reactions along with various alcohols. Exemplary alcoholsinclude but are not limited to: ethanol, n-butanol, n-propanol andi-propanol among others.

In exemplary laboratory batch transesterification reactions, FAME wasmixed with at least one of the alcohols. FAME was also used in a mixtureof two or three alcohols and one of the catalysts, mentionedhereinabove. In an exemplary embodiment, FAME was a mixture of methylpalmitate, methyl stearate, methyl oleate, methyl linoleate and methyllinolenate. The compositions are shown as percent by weight:

-   -   Methyl oleate: 25.9    -   Methyl linoleate: 46.1    -   Methyl linolenate: 13.3    -   Methyl palmitate: 5.4    -   Methyl stearate: 4.4    -   Monoglycerides: 3.24    -   Unknown: 1.66

For the monoglycerides, three different peaks were observed. The valuereported for monoglycerides was a combination of these three peaks. Thisis typical of a soy or canola oil composition. In an exemplaryembodiment, unreacted alcohols from each reaction mixture were removedby vacuum distillation and the final mixture, containing only esters,was evaluated to determine the cloud point. The results for varioustransesterification reactions involving FAME as one of the reactants arereported with respect to Table 1. The cloud point results for variousester mixtures are provided in Table 2.

TABLE 1 Preliminary results for various transesterification reactionsExp. Temp. Reaction % Average No. Alcohol MR Catalyst (° C.) Time Conv*1 Ethanol 6 Amberlyst-15 78 24 h 59 2 i-Propanol 6 Amberlyst-15 80 24 h72 3 n-Propanol 6 Amberlyst-15 98 24 h 75 4 n-Butanol 6 Amberlyst-70 11024 h 49 5 Ethanol 6 Sulfated zirconia 120 24 h 32 6 Ethanol 6Amberlyst-15 120 24 h 65 7 Ethanol 6 No catalyst 120 24 h 5 8 Ethanol 6SBA-15/Ti(acac)₂(IPA)₂ 120 24 h 22 9 Ethanol 6 SBA-15/(butyl)₂Sn(acac)₂120 24 h 44 10 Ethanol 6 SBA-15 120 24 h 7 11 Ethanol 6 SBA-15/Ru(acac)₂120 24 h 12 12 Ethanol 6 SBA-15/AMPTS-TiCl₄ 120 24 h 15 13 Ethanol 6NaOC₂H₅ in EtOH (21 wt 78  6 h 80 %) 14 Ethanol 6 NaOC₂H₅ 78 16 h 55*The average of 5 major methyl-esters of FAME reacted bytransesterification reaction in a stirred batch vessel. Experiment Nos.1-4, 13, and 14 were performed at a pressure of 1 atm. Experiment Nos.5-12 were performed at 120° C. and 60 psia, the vapor pressure ofethanol at that temperature.

TABLE 2 Cloud point for mixed esters Volume Cloud Mixtures Ratio point(° C.) FAME 100:0  2 FAME + Ethyl lactate 90:10 −1 FAME + Ethyl lactate70:30 −3 FAME + Diethyl succinate 90:10 −4 FAME + Diethyl succinate70:30 −4 FAME + FAEE^(a) 34:66 −5 FAME + FAIE^(b) 27:73 −5 FAME +DBS^(c) 90:10 −1 FAME + DBS 70:30 −3 FAME + FABE^(d)  6:94 −4 FAME +FABE + DBS 5:85:10 −5 FAME + FABE + DBS 4:66:30 −7 FAME + FABE + DBS3:47:50 −8 FAME + FABE + DBS + DBE^(e) 5:85:5:5 −4 FAME + FABE + DBS +DBE 4:66:20:10 −6 FAME + FABE + DBS + DBE 3:47:20:30 −11 FAME + FABE +DBS + DBE 3:47:30:20 −12 FAME + FABE + DBS + DBE 3:47:40:10 −12^(a)Fatty acid ethyl esters ^(b)Fatty acid iso-propyl esters ^(c)Dibutylsuccinate ^(d)Fatty acid n-butyl ester ^(e)Dibutyl ether

Cloud point analysis, as in Table 2, of mixed alkyl esters of fattyacids can range from about 5-14° C. (9-25° F.) lower than methyl estersof fatty acids. Therefore, in an exemplary embodiment these mixed esterscan be used as diesel-fuel in not-so-harsh winter conditions with few ifnot any further additives. Since FAME is used as a substrate, the ratioof each alkyl ester in the final mixture can be controlled by the extentof conversion of methyl esters to respective alkyl ester or by blendingthe pure FAME with other pure alkyl esters in various proportions as perthe requirements.

In the exemplary process associated with the present invention, FAME wasused as a substrate to synthesize fatty acids ester of higher alcoholand solid acid catalysts instead of base catalysts. Some of theadvantages associated with these particular exemplary processes includebut are not limited to:

-   -   1) Use of solid acid catalysts eliminates the problems        associated with base catalysts; and    -   2) The use of solid acid catalysts allows the processes to be        carried out on a continuous basis as opposed to only batch        processes, typical with base catalysts.

In an exemplary embodiment, the temperature at which the reaction takesplace with the acid catalysts is from about 50 to 200° C. and thepressure can range from about 0.5 to 20 atmospheres. Operatingconditions are largely dependent upon the chosen catalyst. For example,ion exchange resins are typically best up to 150° C.

Fatty acid esters of other higher alcohols with 100% conversion and 100%purity using FAME as a substrate are produced on a continuous-scale byreactive distillation.

In a further aspect of the present invention the process of synthesizinghigher alkyl fatty acid esters from FAME via transesterification withhigher alcohols with solid acid catalysts, was also a suitable methodfor converting residual triglycerides (along with FAME in biodiesel) toalkyl esters of fatty acids. This helps in maintaining the totalglycerin concentration in the biodiesel at a permissible level as perAmerican Society for Testing and Materials (ASTM) standard.

Thus, the present invention provides a process step in biodieselproduction that removes undesired impurities and provides a mixed esterthat inhibits crystal formation at lower temperature, thus effectivelyeliminating the challenges associated with fuel properties such as cloudpoint and therefore producing a consistent, reliable diesel fuelsubstitute.

The process is shown in FIG. 1. The product stream from a conventionalbiodiesel production facility contains primarily fatty acid methylesters (FAME) but contains small quantities of impurities such astriglycerides (TG), diglycerides (DG), monoglycerides (MG), glycerol(GO), methanol, water, and transesterification catalyst. In conventionalbiodiesel production, these impurities are removed (often incompletely)by a combination of water washing and addition of solid absorbents. Bothmethods lead to questionable results and generation of waste.

As shown in FIG. 1, in an exemplary process associated with the presentdisclosure, a reactive distillation column is provided as an add-on toexisting biodiesel production plants to facilitate purification oftraditional biodiesel production. The FAME+impurities stream is furthertransesterified with ethanol, higher alcohols (C2 or higher) and/or amixture thereof, in the reactive distillation column. This processfurther transesterifies the FAME and the Fatty Acid impurities therebyforming desired product to be collected at the bottom of the column. TheHigh Purity biofuels contains mixed alkyl esters of fatty acids. Theimpurities are essentially removed. In a further exemplary embodiment,any glycerol present in the initial impure FAME stream coming from thebiodiesel process and feeding into the reactive column is removed.Glycerol will leave the column at the bottom along with the high puritymixed alkyl ester product stream. Several techniques can be employed toessentially remove the glycerol including but not limited to: a phaseseparation, washing the biodiesel with water, or by using an adsorbentto remove glycerol. U.S. Pat. No. 7,321,052 to Miller et al., thedescription of which is incorporated by reference in its entiretyherein, describes a process for producing a glycerol acetal useful infuels. U.S. Pat. No. 6,548,681 to Chopade et al, the description ofwhich is incorporated by reference in its entirety herein, describes thefunction and separation of polyol acetals, including glycerol.

In the process diagram shown in FIG. 1, the biodiesel product stream isfed to a reactive distillation column that constitutes an importantaspect of the present invention. The reactive distillation columnconsists of a vertical vessel containing a solid structural packing or adumped packing in which a catalyst material is held. The section of thecolumn containing the acid catalyst material is referred to as thereactive zone. In an exemplary embodiment, the reaction zone comprisesthe catalyst mounted in structured packing elements and supported as asingle unit of the structured packing elements. Liquid moves downward inthe column and vapors, generated via a reboiler at the base of thecolumn, move upward. In the reactive distillation column, the biodieselstream is fed near the top of the reactive zone and flows downward overthe catalyst. An alcohol such as ethanol or a mixture of alcohols suchas ethanol and butanol is added to the column near the bottom of thereactive zone and flows upward as vapor over the catalyst. In thecatalyst zone, several reactions take place. First, the unreacted mono-,di- and tri-glycerides (MG,DG,TG) are transesterified with the alcoholsto produce additional alkyl esters of fatty acids and residual glycerol.This effectively removes these impurities from the fuel stream. Second,a part of the fatty acid methyl esters are transesterified to ethylesters or mixtures such as ethyl+butyl esters. This happens becausethere is free alcohol in the column and because any methanol liberatedin transesterification enters the vapor phase and exits the top of thedistillation column. Third, depending on column temperature someglycerol can undergo etherification with the alcohols present to formglycerol ethers, which themselves are recognized fuel components.

If all reactions take place appropriately, the product from the bottomof the distillation column will be a small quantity of glycerol and highpurity mixed alcohol ester of fatty acids. This mixture constitutes abiofuel with improved properties relative to FAME from conventionalbiodiesel production. The biofuel leaving the bottom of the column canbe washed or treated with absorbent to ensure purity to remove theresidual glycerol, but most often that will not be necessary. Morelikely, a simple phase separation step to remove any unreacted residualglycerol will be carried out and then the fuel can be sent to storagefor use. The stream from the top of the column consists of unreactedethanol or mixed alcohol feed, methanol liberated in transesterificationor stripped from the biodiesel. If the biodiesel contains water, thenwater also leaves through the top of the column. Water is not per segenerated in the transesterification reaction. However, water can beformed in biodiesel production if there are free fatty acids in thestarting triglyceride feedstock. Thus, water may be formed (viaetherification or esterification of free fatty acids) and issubsequently removed through the top of the column with methanol and anyother residual alcohols. These alcohols can be recovered by distillationand recycled into the process. The reaction kinetics are favorable forthe formation of the higher fatty acid esters, as seen in Table 3.

TABLE 3 Rate constants for formation of ethyl, butyl and isoamyl estersof fatty acids by transesterification of methyl esters of fatty acids atvarious temperatures Temp (° C.) Rate constants; k (Kg_(sol) ² · mol⁻¹ ·Kg_(cat) ⁻¹ · s⁻¹ Ethyl oleate Butyl oleate Isoamyl oleate  80 1.5 ×10⁻⁵ 100   3 × 10⁻⁵ 4 × 10⁻⁵  4 × 10⁻⁵ 120  14 × 10⁻⁵ 9 × 10⁻⁵ 11 × 10⁻⁵Ethyl linoleate Butyl linoleate Isoamyl linoleate  80 1.5 × 10⁻⁵ 100   3× 10⁻⁵ 4 × 10⁻⁵  4 × 10⁻⁵ 120  14 × 10⁻⁵ 9 × 10⁻⁵ 11 × 10⁻⁵ Ethyllinolenate Butyl linolenate Isoamyl linolenate  80 2.5 × 10⁻⁵ 100 4.5 ×10⁻⁵ 6 × 10⁻⁵ 5.5 × 10⁻⁵  120 21.5 × 10⁻⁵  12.5 × 10⁻⁵   14.5 × 10⁻⁵  

These rate constants are for the transesterification of methyl esters toethyl, butyl, or isoamyl esters as per the following reaction: Fattyacid methyl ester+alcohol (e.g. ethanol)=Fatty acid alkyl (ethyl)ester+methanol

The catalyst for these reactions is Amberlyst 15 cation exchange resin.These results show that it is kinetically practical to carry outtransesterification of methyl esters to other esters using ion exchangeresins.

A set of experiments were conducted to characterize the equilibriumconstant for the transesterification of methyl esters (e.g. biodiesel)to butyl or ethyl esters. The reactions were carried out in stirredvessels in a laboratory, analogous to a single equilibrium stage in areactive distillation column, for reaction times ranging from 2 hr to 8days. Reactions were considered to be at equilibrium when thecomposition was unchanged over several sample collections. Table 4 showsthe results of the experiments using butanol and ethanol to make butylesters and ethyl esters of fatty acids. The data given in Table 4 showthat a single stage in a reactive distillation column can achieveconversions of methyl ester to butyl ester of approximately 85%.Accordingly, reactive distillation can be an effective and efficientmethod to carry out the desired reactions. By changing the ratio ofalcohol to fatty acid methyl ester, mixed esters of any desiredcomposition via reactive distillation can be produced. These mixedesters are useful as advanced biofuel components.

TABLE 4 Experimental results to measure transesterification equilibriumconstants. (Conditions: alcohol with fatty acid methyl esters (FAMES) in3:1 alcohol:FAME molar ratio; 25 g FAMES in 75 ml Parr reactor, stirrate 1050 RPM.) Catalyst Exp Reaction Quantity Equilibrium No. AlcoholTemp (° C.) Catalyst (wt % FAME) Conversion K_(eq) 1 n-Butanol 100.000Sulfuric acid 0.101 83.753 2.359 2 n-Butanol 100.000 Sulfuric acid 0.19386.713 2.642 3 n-Butanol 100.000 Sulfuric acid 0.250 86.794 2.669 4n-Butanol 120.000 Sulfuric acid 0.168 82.421 2.435 5 n-Butanol 130.000Sulfuric acid 0.149 87.330 2.499 6 Ethanol 100.000 Amberlyst 15 5.00083.190 1.965

The invention constitutes a low-cost, efficient approach to producing ahigh-quality biofuel that meets fuel standards and has improved fuelproperties. The present invention provides for a process for producingmixed esters of fatty acids as biofuel for use in compression ignition(CI) engine, which comprises: (a) partial transesterification of amixture of fatty acid methyl esters with at least one higher alcoholcontaining 2 to 8 carbon atoms in the presence of a heterogeneous solidacid catalyst to produce a mixture of the fatty acid methyl esters andhigher alcohol esters of the fatty acids; and (b) separating thecatalyst from the transesterified reaction mixture. Separation of thealcohol esters of the fatty acids and the FAME can be accomplished usingtraditional separation techniques such as distillation and/orevaporation. The present invention further provides for a biofuelcomposition for use in compression ignition (CI) engine which comprisesa mixture of fatty acid methyl esters and of fatty acid esters of atleast one alcohol containing 2 to 8 carbon atoms. The cloud point of themixture is lower in temperature than that of the fatty acid methylesters alone.

In an exemplary embodiment, the present invention provides for abiofuel, for use in compression ignition (CI) engines, comprisingmixtures of fatty acid methyl esters, fatty acid esters of at least onealcohol containing 2 to 8 carbon atoms, esters of a fermentation derivedorganic acid with at least one alcohol containing 1 to 6 carbon atomsand optionally an ether containing at least 6 carbon atoms as anoxygenate. In an exemplary embodiment, the fermentation derived organicacid can be derived from a carbohydrate (such as sugar) fermentationsource. Oxygenated substances are typically described as those that haveincorporated oxygen in the molecule. The term “Oxygenates” typicallyrefers to fuels or additives containing oxygen. Oxygenates are usuallyemployed as additives to reduce carbon monoxide and carbon particulatesthat are created during the burning of the fuel.

Oxygenates may be based on alcohols or ethers. Some exemplary oxygenatesin use include but are not limited to:

-   -   1) Alcohols: Methanol (MeOH); Ethanol (EtOH); Isopropyl alcohol        (IPA); n-butanol (BuOH); t-butanol; and    -   2) Ethers: Methyl tert-butyl ether (MTBE); Tertiary amyl methyl        ether (TAME); Tertiary hexyl methyl ether (THEME); Ethyl        tertiary butyl ether (ETBE); Tertiary amyl ethyl ether (TAEE);        propyl ether (DIPE); Dipropyl ether; Dihexyl ether; Dioctyl        ether, iso-amyl-ether.

Further exemplary oxygenate related compositions are described in U.S.Pat. No. 6,468,319 issued to Yeh et al., the subject matter of which isincorporated by reference in its entirety herein. Yeh et al. describesdiesel fuel containing ester compositions effective in reducingemissions. U.S. Pat. No. 5,268,008 issued to Kanne describes hydrocarbonfuel compositions containing orthoesters to reduce particulate emissionstherefrom when combusted in an internal combustion engine. Accordingly,the description set forth in Kanne is incorporated by reference in itsentirety herein. The description of commonly owned U.S. application,2006/0014977, filed Jul. 19, 2004 is incorporated by reference herein inits entirety.

While the present invention is described herein with reference toillustrated embodiments, it should be understood that the invention isnot limited hereto. Those having ordinary skill in the art and access tothe teachings herein will recognize additional modifications andembodiments within the scope thereof. Therefore, the present inventionis limited only by the claims attached herein.

1. A process for preparing a fatty acid ester mixture useful, alone orin combination with a petroleum fuel, in a compression ignition engine,which comprises: (a) reacting an impure fatty acid methyl ester mixturewith at least one alkyl alcohol containing 2 to 8 carbon atoms in thepresence of a base or acid, wherein the impure fatty acid methyl estermixture comprises fatty acid methyl ester and methanol, one or more of amonoglyceride, a diglyceride, and a triglyceride as impurities; whereinthe reaction partially transesterifies the impure methyl ester mixtureto produce a mixture of fatty acid alkyl esters containing 1 to 8 carbonatoms; and (b) removing methanol formed in the reaction and unreactedalkyl alcohol to produce the fatty acid ester mixture.
 2. The processaccording to claim 1, wherein at least one alkyl alcohol is ethanol andat least one of the produced alkyl fatty acid esters is a fatty acidethyl ester.
 3. The process according to claim 1, wherein the impuremixture is reacted with a mixture of two or more alcohols at least oneof which is ethanol.
 4. The process according to claim 3 wherein themixture of alcohols comprises ethanol and butanol and the reaction withthe impure fatty acid methyl ester mixture further produces a fatty acidethyl ester and a fatty acid butyl ester.
 5. A process for producingmixed esters of fatty acids, which comprises: (a) partiallytransesterifying at a temperature between about 50 and 200° C. and apressure between about 0.5 and 20, a mixture of fatty acid methyl esterswith at least one alkyl alcohol containing 2 to 8 carbon atoms in thepresence of a heterogeneous solid acid catalyst to produce a mixture ofthe fatty acid methyl esters and alkyl alcohol derived fatty acidesters; and (b) separating the esters from the catalyst.
 6. The processof claim 5 wherein the mixture of fatty acid methyl esters is comprisedof methyl palmitate, methyl stearate, methyl oleate, methyl linoleateand methyl linolenate.
 7. The process of claim 5 wherein the alkylalcohol is selected from the group consisting of ethanol, n-propanol,n-butanol, iso-amyl alcohol and mixtures thereof.
 8. A continuousprocess for producing a fatty acid alkyl ester mixture which comprises:(a) countercurrently transesterifying a fatty acid methyl ester at atemperature between about 50 and 200° C. and a pressure between about0.5 and 20 atmospheres with at least one alkyl alcohol having 2 to 8carbon atoms in the presence of a solid acid catalyst in a reactive zonein a distillation column to produce a fatty acid alkyl ester mixture;and (b) removing the produced fatty acid alkyl ester mixture at thebottom of the column and the alcohol from the top of the column.
 9. Theprocess according to claim 8, wherein the reactive distillation columncomprises: (a) the reactive zone containing the solid acid catalyst; (b)a fatty acid ester inlet above the catalyst; (c) an alcohol inlet belowthe catalyst; (d) product outlet at the bottom of the column forseparating the produced fatty acid alkyl ester; and (e) an outlet at thetop of the column for methanol formed in the reaction and at least onealcohol which is unreacted.
 10. The process according to claim 8,wherein the solid catalyst is a heterogeneous solid acid catalystmounted in structured packing elements and supported as a single unit ofthe structured packing elements.
 11. The process according to claim 8,wherein the fatty acid methyl ester comprises a mixture of: (i) fattyacid methyl esters, and (ii) methanol, monoglyceride, diglyceride,triglyceride, and glycerol as impurities.
 12. The process according toclaim 11, wherein at least the monoglyceride, diglyceride, andtriglyceride of the impurities are transesterified in the reaction withthe alkyl alcohol having 2 to 8 carbon atoms producing mixed fatty acidalkyl esters to be removed at the bottom of the column, along withglycerol.
 13. The process according to claim 11, wherein the fatty acidmethyl ester is generated from a preliminary transesterificationreaction of a vegetable oil as a triglyceride with methanol.
 14. Theprocess according to claim 13, wherein the preliminarytransesterification reaction of a vegetable oil as a triglyceride withmethanol takes place in a biodiesel production plant and the reactivedistillation column is added onto the biodiesel production plant toreceive the fatty acid methyl ester product stream.
 15. The processaccording to claim 8, wherein the fatty acid alkyl ester produced is inadmixture with the fatty acid methyl ester.
 16. The process according toclaim 8, wherein the alkyl alcohol is ethanol and the fatty acid alkylester is a fatty acid ethyl ester.
 17. The process according to claim 8,wherein the alkyl alcohol is a mixture of alcohols having 2 to 8 carbonatoms.
 18. The process according to claim 17, wherein the alkyl alcoholmixture contains ethanol and butanol and the fatty acid alkyl esterproduced comprises a mixture of a fatty acid ethyl ester and a fattyacid butyl ester.
 19. The process according to claim 8, wherein thealkyl alcohol is selected from the group consisting of ethanol,propanol, isopropanol, butanol, isobutanol, amyl and iso-amyl alcoholand mixtures thereof.
 20. The process according to claim 8, wherein theproduced fatty acid alkyl ester has a cloud point lower in temperaturethan the methyl fatty acid ester.
 21. The process according to claim 8,wherein the produced fatty acid alkyl ester is in addition blended witha petroleum diesel fuel.
 22. The process according to claim 21, whereinthe petroleum diesel fuel is 50 to 95% of the blend.
 23. The processaccording to claim 8, wherein the fatty acid methyl ester is selectedfrom the group consisting of methyl palmitate, methyl stearate, methyloleate, methyl linoleate, methyl linolenate, and mixtures thereof. 24.The process according to claim 23, wherein the produced fatty acid alkylester is derived from acids selected from the group consisting ofstearic acid, oleic acid, linoleic acid, linolenic acid, and mixturesthereof.
 25. A composition useful in a compression ignition (CI) enginewhich comprises a mixture of fatty acid methyl esters and of fatty acidalkyl esters of at least one alkyl alcohol containing 2 to 8 carbonatoms, wherein the cloud point of the mixture is lower in temperaturethan that of the mixture of the fatty acid methyl esters alone.
 26. Thecomposition of claim 25 wherein the mixture of fatty acid methyl esterscomprises esters of palmitic acid, stearic acid, oleic acid, linoleicacid and linolenic acid from soybean.
 27. The composition of claim 25further comprising esters of a fermentation or non-fermentation derivedorganic acid with at least one alkyl alcohol containing 1 to 6 carbonatoms and optionally an ether containing at least 6 carbon atoms as anoxygenate.
 28. The composition of claim 27 wherein the mixture of fattyacid methyl and alkyl esters further comprises esters of dimethylsuccinate.
 29. The composition of claim 27 wherein the fermentation ornon-fermentation derived organic acid ester is derived from an acidselected from the group consisting of lactic acid, succinic acid andmixtures thereof.
 30. The composition of claim 27 wherein thefermentation or non-fermentation derived organic acid ester is derivedfrom an acid selected from the group consisting of propionic acid,butyric acid, isobutyric acid, dicarboxylic and carboxylic acids, andmixtures thereof.
 31. The composition of claim 27 wherein the ether isselected from the group consisting of dibutyl ether (DBE); methyltert-butyl ether (MTBE); tertiary amyl methyl ether (TAME); tertiaryhexyl methyl ether (THEME); ethyl tertiary butyl ether (ETBE); tertiaryamyl ethyl ether (TAEE); propyl ether (DIPE); dipropyl ether; dihexylether; dioctyl ether, and iso-amyl-ether.
 32. The composition of claim27 wherein the oxygenate is an ester.